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Rebuilding a Laptop Battery

by Phil Hughes

on October 5, 2008

When your laptop battery is about to give up the ghost, you are
probably thinking of the typical three alternatives. The most
obvious, of course, is "wow, what a great justification for buying
a new laptop". With laptop prices falling and no such luck with
battery prices, this almost makes sense. But, most of us will
probably just bite the bullet and buy a new battery. The third
alternative would be to replace the cells in the battery you
already have.

This article is about the third alternative. If you are not
fairly skilled in working on electronics, this is not necessarily a
good alternative. But, it is possible. Personally, I was inspired
to do this the first time because the small (3-cell) battery for my
ASUS laptop was very ill and I couldn't find a replacement.

Financially, it also looked like a good choice. The out of stock
new battery cost $129. I found the cells for a bit over $5 each.
So, I ordered the cells and, expecting to have a success, ordered
six more for a sick battery for one of my T23 ThinkPads.

The first trick is to open the old battery without destroying
the plastic or anything important inside. What you need to do is
find where the two parts are glued together and work on the glue
line with something sharp until you can open the battery.

In the case of ThinkPad batteries, there are labels over parts
of the glue line. So, first cut through the labels with a sharp
knife. I then chose to use a wood chisel to first find parts of the
plastic that will flex because there is open space behind them and
then start opening in these areas. You may have better luck with a
utility knife.

Once you get the opening process started, a utility knife works
best in some parts, the wood chisel in others. Just be careful not
to cut too deep or you could damage circuitry or wiring inside.
Also, remember that the goal is to be able to glue the battery case
back together and have it fit in the laptop so cutting out a little
plastic (possibly with a fine saw) may be better than an attempt to
just open it up with the result of warping or breaking the
plastic.

Once you have the battery pack open, it is analysis time. The
most common battery packs contain three, six or nine Li-ion cells
of size 18650. Typically they are in strings of three in series
(that gives you about 11V) and then each string is paralleled. But,
that may not be the case. Do a bit of analysis before you start
cutting wires.

Note that the cell size is not something that indicates that
there are at least 18,649 other choices. It is actually the
diameter of the cell in millimeters (18) followed by the length in
millimeters (650). The big advantage with this system is that if
the cell isn't marked with a size, a metric ruler can help you
out.

As Li-ion batteries need special "care and feeding", you will
find a circuit board within the battery pack, Wires will go from
the circuit board to each cell junction so that the voltages can be
monitored. Also, there will be a thermistor and a thermal circuit
breaker within the pack.

There are lots of places that sell replacement Li-ion cells. One
is Battery
Junction. On their site you will find all the common battery
sizes. They also have both PC boards to protect sets of cells and a
special 18650 cell with a built-in protection circuit. Assuming you
are replacing cells in an existing pack, you just need to buy
regular cells. One thing you do need to watch for, however, is
tabs. Typically you can ask to have solder tabs spot welded to the
cells. This is the right way to go rather than trying to solder
directly to the cell which could easily damage the seals.

Once you have all the pieces, it is time to actually build the
new battery pack. I cannot empathize enough that an error here can
be dangerous. First, you are dealing with batteries that if abused,
can explode or burn. Additionally, the monitor circuit is always
active so you are working on operating electronics. Consider
yourself warned.

I have found the best way to build the new pack is first take
all the cells, the circuit board and the connector out of the
plastic case. Figure out how things are wired. That is, which cells
are in parallel, which are in series and, most important, which is
the positive and which is the negative end of each cell. There is
also usually a thermistor (it looks like a little lump on the end
of two wires) and a thermal breaker attached to the batteries.
Figure this out and, better still, take a picture of it.

In the T2x series ThinkPad batteries, things are quite well
organized. The black wire goes to the negative end of the cell
string. The red and orange together (one is the thermal breaker) go
to the most positive. White goes to the 3.7V tap and yellow to the
7.4V tap.

You are most likely going to have to solder the battery tabs
together or connect them with short pieces of wire. Try to make the
connections as compact as possible. While everything may appear
fine, when you try to put the top on the battery pack you may
discover you need to re-solder connections and/or file down any
lumps.

I start building the new pack within one half of the battery
pack case. Double-stick tape will hold the cells in place as you
work on the battery. Hot glue can be used to stick cells together
and hold down the thermal breaker and thermistor.

When you get all the cells in place, start soldering the wires
from the circuit board. Once again, remember that the circuit is
live so touching a wire to the wrong battery terminal can mean so
long circuit.

Once all the connections are made, it is a good idea to check
the voltages to see if they make sense. Starting at the negative
end, each cell or set of parallel cells should add about 3.7 volts
to the total. The exact voltage will depend, of course, on the
state of charge of the cells but a fairly fresh cell should read
3.5 volts or more with no load.

If everything checks out, put the top on the battery case. If it
doesn't fit, file, adjust, re-solder and such until it does. It is
then a good idea to test the battery in the laptop. A bit of tape
can be used to hold the battery together for testing.

If all goes well, you need to glue the two battery halves back
together. While model cement can be used, I prefer a hot glue gun.
Work slowly, preparing one side or part of a side, holding it
together and let the glue set. Then move along until the entire
battery is sealed. Then, with a knife, cut off any glue that
extends beyond the seams.

That's it. You just proved you could save $100 by spending half
a day playing with hand tools and glue.

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